1. Field of the Invention
The present invention is in the field of controls, specifically the rejection of unwanted mechanical disturbances and more particularly to the rejection of disturbances which change frequency with time.
2. Description of the Prior Art
Vibration nulling (or disturbance rejection) systems for use on spacecraft which are positioned so as to maintain a fixed pointing direction (for example a telescope) are known. In such a situation, the spacecraft needs to maintain the desired pointing direction even though forces from inside or outside the vehicle may cause disturbances. For example, when a space vehicle moves from a position in which it is in shadow, into a position in which it is in sunlight, the temperature difference can suddenly cause parts of the spacecraft to snap or sharply bend and, in so doing, produce vibrations that, if uncompensated, could cause the spacecraft to vibrate, thereby affecting the pointing direction for an undesirably long period of time before damping out. Heretofore, such unwanted vibrations have been calculable and their frequency constant so that a vibration filter can be introduced into the spacecraft torque actuation controls (e.g. torque actuators such as control moment gyros (CMGs), reaction wheels, torquing bars or thrusters etc.), which quickly nulls or cancels the unwanted frequency. Such a circuit is described in a paper entitled "Control Synthesis For Flexible Space Structures Excited by Persistent Disturbances," presented by Bong Wie and Marcelo Gonzalez in the Journal of Guidance, Control and Dynamics, Vol. 15, No. 1, 1992, pp 73-80.
Unfortunately, while some disturbances produce fixed frequency vibrations, other disturbances produce vibrations that change frequency with time. For example when a spacecraft is using a mechanical arm or grapple to move some object closer to or further away from the spacecraft, the inertia of the mechanical arm and the object changes as the effective length of the arm changes. Consequently, the frequency of any disturbance produced by the arm and object changes with time during the maneuver. Similarly, the motors which drive actuators, such as CMGs, may introduce cogging forces as the poles pass over the coils and the frequency of this kind of disturbance also varies with time since the speed of rotation of the actuators are not constant. Many other types of disturbances may cause variable frequency vibrations and the time to allow these vibrations to null themselves out may be unacceptably long. In such situations, the filters of the prior art are not sufficient to quickly null out the undesirable vibrations.